1. Field of the Invention
The present invention relates to a fixing controller for an induction-heating fixing device, an electrophotographic image forming apparatus, such as a copying machine, a printer, or a facsimiles machine, that is equipped with the fixing controller, and a fixing control method.
2. Description of the Related Art
In an image forming apparatus of the above mentioned type, a heat-roller fixing device is normally employed. In a heat-roller fixing device, each paper sheet having an unfixed toner image formed thereon passes through between a heating fixing roller and a pressure roller. The fixing roller and the pressure roller heat and press the unfixed toner image, thereby fixing it onto the paper sheet.
Such a heat-roller fixing device is normally equipped with a halogen lamp as a heater in the fixing roller, and the halogen lamp heats the inside of the fixing roller.
With such a heating technique utilizing a halogen lamp or the like, however, a long period of time is needed to heat the fixing roller to a required temperature, and heat loss in the heater is also large. In view of this, there has been a strong demand for an efficient fixing device with a short warm-up time.
Meanwhile, an induction-heating (hereinafter also referred to as “IH”) fixing device can heat a fixing roller instantly with an eddy current through electromagnetic induction, and the warm-up time can be dramatically shortened. Such an IH fixing device is disclosed in Japanese Laid-Open Patent Application No. 2001-175118 (hereinafter referred to as Patent Document 1), for example.
However, the IH fixing technique requires a fine control operation, because the temperature rising speed is much higher than that of the halogen-heater fixing technique. As a result, the control workload on the central processing unit (CPU) that performs the entire control operation becomes very large. Conventionally, the CPU that controls an entire image forming apparatus also serves as the IH controller. With such a structure, there are problems in that the high-performance CPU leads to a cost increase and the control software becomes very complicated (with a higher probability of bugs). If a regular CPU is employed to avoid those problems and to simplify the IH control operation, however, another problem is caused with the insufficient induction-heating control.
So as to counter those problems, Japanese Patent Publication No. 3400402 (Patent Document 2) discloses a 2-CPU fixing technique. By this 2-CPU fixing technique, a CPU is mounted in an induction-heating control circuit, and this CPU is independent of the CPU that controls the entire image forming apparatus.
By the technique disclosed in Patent Document 2, however, the CPU of the main control circuit controls the CPU of the induction-heating control circuit, using a power control signal and a heating ON/OFF signal. In such a structure, the workload on the CPU of the main control circuit cannot be sufficiently reduced. More specifically, the CPU of the main control circuit in this structure needs to perform arithmetic operations for the power control signal and the heating ON/OFF signal, based on the temperature of the fixing unit detected by a thermistor. In this structure, the CPU of the induction-heating control circuit merely serves as a mediator between the CPU of the main control circuit and the inverter circuit. Therefore, the workload on the CPU of the main control circuit cannot be sufficiently reduced, and the control software cannot be prevented from becoming complicated.
With the 2-CPU fixing technique, on the other hand, the CPU of the induction-heating control circuit directly performs power control on the excitation coil. In doing so, it is necessary to take safety measures against a runaway condition in the CPU of the induction-heating control circuit. In the prior art, however, this aspect is not taken into consideration at all. Since the IH fixing technique involves very rapid temperature rises, there is a greater danger than in a halogen-lamp fixing operation, for example, of the CPU of the IH control circuit going into a runaway condition. Therefore, it is very important to take appropriate safety measures. Japanese Laid-Open Patent Application No. 2002-174982 (Patent Document 3) discloses a method of forcibly cutting off the high-frequency current supply, utilizing a temperature detection signal that indicates a temperature error caused in a CPU runaway condition. However, this method merely employs a conventional temperature error detecting technique, and a runaway condition in the CPU is not actually detected by the method. As a result, a time lag is caused, and prompt measures cannot be taken against a runaway condition.